JPH0443770A - Adaptive clamp circuit for ghost eliminating device - Google Patents

Abstract

PURPOSE:To obtain a clamp circuit able to correctly recover a DC component of a video signal by providing a means discriminating whether the synchronization is stable or unstable and a means selecting a video signal subject to pedestal clamp with a vertical keyed pedestal clamp circuit when the stable synchronization is discriminated. CONSTITUTION:A video signal Sin given to an input terminal 1 is divided into two systems via a buffer 14 and an amplifier 17, the one is led to a SYNC tip clamp circuit 11 via a buffer 15 and the other is led to a V keyed pedestal clamp circuit 12 via a buffer 16. When a horizontal synchronizing signal and a vertical synchronizing signal included in a received television signal and a basic clock in a ghost eliminating device are in a prescribed relation the synchronization is discriminated to be stable. Based on the result of discrimination, the video signal pedestal-clamped by the V keyed pedestal clamp circuit 12 is selected and the result is outputted as a video signal after clamping. Thus, the DC component of the video signal is correctly recovered regardless of occurrence of a ghost fault.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a clamp circuit used before A/D conversion in a digital ghost removal device in a television receiver.

Fig. 5 shows an example of a conventional clamp circuit (hereinafter referred to as the ``conventional example''), and Fig. 6 shows waveforms to explain the operation of this clamp. This is a general pedestal clamp circuit, which regenerates the DC component of the video signal by keying the pack pouch of the horizontal synchronizing signal.In this clamp circuit, the reclamper is operated using transistors (TR54) (TR55), capacitor (C51), etc. configured.

Then, the video signal (Sin) applied to the input terminal (1)
The signal is passed through a buffer consisting of a transistor (TR51), etc., and then amplified by an amplifier consisting of a transistor (TR52) (TR53), etc.
R54) is added to the base. On the other hand, by applying the horizontal synchronization signal (8N) (Fig. 6 (b)) synchronously separated from the input video signal (Sin) by the horizontal synchronization separation circuit (51) to the monostable multivibrator (MM51) (MM52), As shown in FIG. 6(c), a horizontal clamp pulse (SHC) located at the back porch is obtained. This horizontal clamp pulse (SHc) is applied to a transistor (TR5).
6), etc., and the switching transistor (TR57) is turned on during the period of the horizontal clamp pulse (SHQ) by the output of the buffer. here,
The emitter of the switching transistor (TR57) is grounded via the capacitor (C52) and a predetermined reference voltage is applied by the reference voltage generation circuit (52), and the collector is connected to the transistor (C51) of the capacitor (C51).
It is connected to the terminal on the TR55) side. Therefore, the bank porch level of the horizontal synchronizing signal in the video signal is clamped to a predetermined reference voltage, and the transistor (TR5
5) The video signal after clamping obtained at the emitter (So
ut) is output from the output terminal (2).

If we consider the operation of the above clamp circuit when a ghost disturbance occurs, for example, in the case of a long ghost with a long delay time, the video signal (interfering signal) corresponding to the ghost will also affect the back porch of the horizontal sync signal. Because of this, the clamp circuit may not operate properly. In other words, the original video signal and the video signal corresponding to the ghost (interfering signal) overlap at the back porch, and in the worst case,
It becomes impossible to clamp the pedestal level of the original video signal. Furthermore, if the video signal that is ghost-proof is a moving image, the clamp level will also vary depending on the moving image.

By the way, the video signal input to the ghost removal device is
Since it is assumed that a video signal (interfering signal) corresponding to ghost is added, using a conventional clamp circuit that keys the back porch of the horizontal synchronization signal will cause the above problem, and the DC minutes may not be played correctly. If the DC component cannot be reproduced correctly, the relative level of the video signal will fluctuate with respect to the reference signal for ghost removal (hereinafter referred to as "rGCR signal") included in the video signal (vertical synchronization period). As a result, the ghost removal device does not work properly. It is also assumed that the ghost removal device will be used in situations where stable synchronization cannot be obtained, such as in poor reception conditions (at the time of a weak electric field).

Therefore, the present invention performs stable clamping even under adverse conditions where synchronization is not stable, and when stable synchronization is obtained, video signals of various levels or delay times corresponding to ghosts are added. Another object of the present invention is to provide a clamp circuit that can correctly reproduce the DC component of a video signal.

In order to achieve the above object, the present invention provides a clamp circuit for regenerating the DC component of a video signal used in a ghost removal device of a television receiver, which includes a sync chip clamp circuit and a sync chip clamp circuit immediately before or after a vertical synchronizing signal. a V-keyed pedestal clamp circuit that clamps the blanking level of the ghosting pulse; and whether there is a predetermined relationship between the horizontal synchronization signal and vertical synchronization signal separated from the received television signal and the basic clock in the ghost removal device. determining means for determining whether the synchronization is stable or unstable depending on whether the synchronization is stable or unstable; and a selection means for selecting the video signal pedestally clamped by the V-keyed pedestal clamp circuit when the synchronization is determined to be stable.

Work-1- According to such a configuration, if the horizontal synchronization signal and vertical synchronization signal included in the received television signal and the basic clock in the ghost removal device do not have a predetermined relationship, it is determined that the synchronization is unstable. be done.

Then, based on this determination result, the video signal clamped by the sync tip clamp circuit is selected and output as a clamped video signal. On the other hand, if the horizontal synchronization signal and vertical synchronization signal included in the received television signal and the basic tarok in the ghost removal device have a predetermined relationship, it is determined that the synchronization is stable. Based on this determination result, the video signal pedestally clamped by the keyed pedestal clamp circuit is selected and output as the clamped video signal.

The thus output clamped video signal is used for ghost removal processing.

1 example - Embodiments of the present invention will be described below with reference to the drawings.

FIG. 1 is a block diagram showing the configuration of an adaptive clamp circuit for a ghost removal device according to the present invention. In this figure, the video signal (Sin) applied to the input terminal (1) passes through a buffer (14) and an amplifier (17), and then is divided into two systems, one of which passes through a buffer (15) and is output to a single chip clamp. The other is led to a V-keyed pedestal clamp circuit (12) through a buffer (16). And the sync tip clamp circuit (11) and ■
The outputs from both of the keyed pedestal clamp circuits (12) are applied to the changeover switch (13), and the output of one of the clamp circuits is selected by the clamp mode changeover signal (S+a), and the clamped video signal (Sout )
It is output from the output terminal (2) as . The clamp mode switching signal (Sm) is generated by a circuit as shown in FIG. 7, as will be described later, and this circuit corresponds to a means for determining whether synchronization is stable or unstable.

FIG. 2 is a circuit diagram showing a specific example of an adaptive clamp circuit for a ghost removal device embodying the present invention.

Further, FIGS. 3 and 4 are waveform diagrams for explaining the operation of the present clamp circuit. The configuration and operation of this circuit will be explained below based on FIGS. 2 to 4.

In Figure 2, the video signal (
Sin) is a buffer (14) consisting of a transistor (TR21) etc., a transistor (TR22) (TR
After passing through an amplifier (17) consisting of a transistor (TR24), etc., and a buffer consisting of a transistor (TR24), etc., it is divided into two systems, one of which passes through a buffer (15) consisting of a transistor (TR25), etc., and is connected to a sync chip clamp circuit (
11), and the other is led to a V-keyed pedestal clamp circuit (12) through a buffer (16) consisting of a transistor (TR27) and the like.

The sink chip clamp circuit (11) includes a transistor (
Diode (D21) placed after TR25),
Capacitor ((:21) (C24), and resistor (
VR22) (R21), and clamps the video signal to the DC potential set by the resistor (VR22) (R21). In general, a sync tip clamp circuit does not require a clamp pulse, so compared to a keyed type pedestal clamp circuit, it can perform normal lamp operation even in situations where synchronization is unstable due to poor reception conditions. Therefore, the video signal that has been stably clamped is sent to the analog switch (AS21) through a buffer made of a transistor (TR26) and the like.

On the other hand, ■ Keyed pedestal clamp circuit (12)
is composed of transistors (TR27) to (TR30), capacitors (C22) (C23), etc.
The blanking level of the fusing pulse immediately before or after the vertical synchronization signal is clamped to a predetermined potential given by the reference voltage generation circuit (22). In other words, the key vertical clamp pulse (Suc) is generated by the vertical synchronization separation circuit (21
) by adding the vertical synchronization signal (SU) (Fig. 3 (b)) synchronously separated from the input video signal (Sin) to the monostable multivibrator (MM21) (MM22), the vertical synchronization signal (S, ) is Immediately before or immediately after (Figure 3 (c)
) occurs during the equalization period (T2) or (T4). This vertical clamp pulse (Suc) is a transistor (TR29).
) etc., and a transistor (TR
30), the capacitor (C23), and the reference voltage generation circuit (22) perform the same operations as in the conventional example described above, so that the blanking level of the equalization pulse immediately before or after the vertical synchronization signal (SU) is set to a predetermined value. Clamped to potential. For this reason, ■ Keyed pedestal clamp circuit (12)
In this case, the video signal is clamped at a position far away on the time axis from the OCR signal and the video signal portion (horizontal scanning period), so that even if a ghost problem occurs, the DC component can be correctly reproduced. Therefore, regardless of the occurrence of the ghost failure, a video signal whose DC component is correctly reproduced is output from the emitter of the transistor (TR2g) and sent to the analog switch (AS22).

The analog switch (AS21) (AS22) is controlled by the clamp mode switching signal (Sm), and when the clamp mode switching signal (Sm) is low level, it selects the video signal clamped by the sync chip clamp circuit (11) and switches it to high level. When the level is selected, the video signal clamped by the keyed pedestal clamp circuit (12) is selected. The video signal selected in this manner is output from the output terminal (2) as a clamped video signal (Sout).

Here, the clamp mode switching signal (Sm) is the well-known O
This signal indicates whether or not the relationship between the horizontal synchronization signal, the vertical synchronization signal, and the basic clock is normal in the CR gate generation circuit, and can be generated by a circuit as shown in FIG. 7, for example. In the circuit shown in FIG. 7, a PLL type clock generation circuit (72) generates a basic clock (4fsc) with a frequency four times the color subcarrier frequency, and this basic clock (4fsc) is divided by a countdown circuit (73). By doing so, an internal horizontal synchronizing signal (IHD) and an internal vertical synchronizing signal (IVD) are obtained. In other words, the internal horizontal synchronization signal (IHD) is 1/1/1 of the basic clock (4fsc).
The internal vertical synchronization signal (IV
D) is the basic clock (4fsc) divided into 17910 and then further divided into 1/262.5. These internal synchronization signals (IHD) and (IVD) are a horizontal synchronization signal (3N) and a vertical synchronization signal (SU) separated from the video signal (Sin) by an analog synchronization separation circuit (71).
) and are compared by comparators (74) and (75), respectively, and if they match, a high level is output, and if they do not match, a low level is output. And AND gate (76)
By taking the AND of the output signals of these comparators (74) and (75), the clamp mode switching signal (S+a) is obtained.
I am getting . Therefore, when this clamp mode switching signal (Sm) is at high level, the synchronization is stable and the OC
It can be considered that the R signal is in a state where the R signal can be reliably captured, and when it is at a low level, the synchronization is unstable and the OCR signal cannot be reliably captured. By the way, as mentioned above, when the clamp mode switching signal (Sm) is at low level, the video signal clamped by the sync chip clamp circuit (11) is selected, and when it is at high level, the keyed pedestal clamp circuit (12) is selected. The video signal clamped by is selected. Therefore, from the output end (2),
Even under adverse conditions where synchronization was unstable, a video signal with stable clamping was output, and when stable synchronization was obtained, the DC component was reproduced correctly regardless of the occurrence of ghost disturbances. A video signal is output.

As explained above, according to the present invention, when synchronization is unstable due to a weak electric field, stable clamping is performed on the video signal, and when stable synchronization is obtained, clamping is performed stably. To correctly reproduce the DC component of a video signal regardless of the occurrence of a ghost disturbance.

In addition, if stable synchronization is obtained, clamping can be performed to minimize nonlinear distortion of the waveform. For example,
Assuming that the video signal shown in FIG. 4(A) is input, in this case, the waveform shown in FIG. 4(D) is obtained as the video signal waveform after clamping.

This waveform is a waveform clamped by the keyed pedestal clamp circuit, so the lower part of the horizontal sync signal is cut in the sync tip clamp (the part indicated by the arrow in figure (0)), and the horizontal sync signal is cut in the conventional pedestal clamp. The distortion that occurred on the back pouch or front pouch (the position of the clamp pulse) (the part indicated by the arrow in the same figure (c)) is not seen.

Therefore, according to the present invention, by performing appropriate ramping in the ghost removal device according to the state of the received signal, it is possible not only to operate the ghost removal device normally but also to perform highly accurate ghost removal.

[Brief explanation of the drawing]

FIG. 1 is a block diagram showing the configuration of an adaptive clamp circuit for a ghost remover according to the present invention, and FIG. 2 is a circuit diagram showing a specific example of an adaptive clamp circuit for a ghost remover embodying the present invention. 3 and 4 are waveform diagrams for explaining the operation of the adaptive clamp circuit for the ghost removal device, FIG. 5 is a circuit diagram showing an example of a conventional clamp circuit, and FIG. 6 is a conventional clamp circuit. FIG. 7 is a block circuit diagram showing an example of a circuit for creating a clamp mode switching signal. (1)...Input end. (2)...Output end. (11)...Sync tip clamp circuit. (12)...V-keyed pedestal clamp circuit. (13)...Changing switch (selection means). (Sin)...Input video signal. (Sout)...Output video signal (video signal after clamping). (Sm) ...Clamp mode switching signal. (sue) Vertical clamp pulse. (SH) ...Horizontal synchronization signal. (Su) ...Vertical synchronization signal. (4fsc)...Basic clock. (T2) (T4)... Equalization period. (T3) ...Vertical synchronization period.

Claims (1)

[Claims] (1) In a clamp circuit that reproduces the DC component of a video signal used in a ghost removal device of a television receiver, a sync chip clamp circuit and a blanking level of the equalization pulse immediately before or after the vertical synchronization signal are clamped. Whether the synchronization is stable depends on whether or not there is a predetermined relationship between the V-keyed pedestal clamp circuit, the horizontal synchronization signal and vertical synchronization signal separated from the received television signal, and the basic clock in the ghost removal device. determining means for determining whether the synchronization is unstable; and selecting a video signal clamped by the sync chip clamp circuit when the determining means determines that the synchronization is unstable; and determining that the synchronization is stable by the determining means; An adaptive clamp circuit for a ghost removal device, comprising: selection means for selecting a video signal pedestally clamped by the V-keyed pedestal clamp circuit when the V-keyed pedestal clamp circuit is used.